One of the problems encountered when using membranes in lithium-polymer batteries is with respect to their degradation over time. This phenomenon is particularly important when using radical crosslinking agents activated chemically or by photochemistry during the formation of a crosslinked membrane to increase its mechanical strength. Indeed, the formation of free radicals in the polymeric structure causes degradation of the membrane by extraction of the protons present on the main chain, thereby causing chain scission mechanism. This phenomenon is particularly important during the use of UV light for initiating the reaction (see Koo, G.-H. et al, J. Appl. Polym. Sc. 2012, 125 (4), 2659-2667; and Shyichuk, A. V. et al, Polymer Degradation and Stability 2005, 88 (3), 415-419).
Chemical crosslinking by reaction of an amine with an oxirane group is common in the world of epoxy-type coatings and draws more and more of the attention of chemists due to its great reactivity (see Benaglia, M. et al, Polym. Chem. 2013, 4 (1), 124-132; and Gadwal, I. et al, Macromolecules, 2013, 47 (1), 35-40). These materials, commonly called thermosetting materials, are of interest because of their hardening which increases both their chemical and mechanical resistance (see Enns, J. B. et al, J. Appl. Polym. Sc., 1983, 28 (9), 2831-2846; and Jordan, C. et al, J. Appl. Polym. Sc., 1992, 46 (5), 859-871).
A literature review shows that there are several types of chemical crosslinking reactions. Most lead to the formation of unwanted by-products such as water or acids. Other reactions are carried out at high temperature or in the presence of a copper-based inorganic catalyst (for example, see Tillet, G. et al, Prog. Polym. Sc., 2011, 36 (2), 191-217). These various residual elements are not desirable in an electrochemical system.
Several amine crosslinking agents are present on the market. However, these are not very volatile and remain trapped in the polymer, the crosslinking reactions not being 100% completed in order to obtain optimum mechanical properties (see Jordan et al, Supra).
It has been demonstrated that the use of a Jeffamine®-type crosslinking agent for the formation of gel-like membranes for the ionic conduction of lithium allows for the formation of a crosslinked polymer (see, for example, Luo, D. et al, J. Appl. Polym. Sc. 2011, 120 (5), 2979-2984). It is however possible that these free crosslinking agents react during battery cycling thereby causing an accelerated degradation of the lithium anode and/or of the gel electrolyte by migration of compounds from the membrane to the anode. Moreover, the presence of free Jeffamine increases the elasticity of the membrane and reduces its mechanical properties by a plasticizer effect.